1. Field of the Invention
The present invention relates to a reduction casting method. More particularly, the invention relates to a reduction casting method that eliminates a state in which a molten metal is not fully filled into the cavity and is capable of shortening a casting cycle.
There are various types of casting methods such as a gravity casting method (GDC), a low pressure die casting method (LPDC), a die casting method (DC), a squeeze casting method (SC) a thixomolding method. All of these methods perform casting by pouring molten metal into a cavity of a molding die, thereby molding the thus-poured molten metal into a predetermined shape. Among these casting methods, in a method in which an oxide film is likely to be formed on a surface of the molten metal, for example, at aluminum casting or the like, a surface tension of the molten metal is increased by the oxide film formed on the surface of the molten metal to deteriorate a flowing property, a running property and an adhesive property of the molten metal thereby causing problems of casting imperfections such as insufficient filling, a surface fold and the like.
As a method to solve these problems, the present applicant has proposed a reduction casting method which is capable of performing casting by reducing an oxide film formed on a surface of a molten metal (for example, JP-A-2000-280063). In this reduction casting method, a magnesium-nitrogen compound (Mg3N2) having a strong reducing property is produced by using a nitrogen gas and a magnesium gas and, then, the thus-produced magnesium-nitrogen compound is allowed to act on the molten metal of aluminum, thereby performing casting. By pouring the molten metal into a cavity of a molding die in a state in which the magnesium-nitrogen compound is deposited on a surface of the cavity of the molding die, when the molten metal comes into contact with the surface of the cavity, an oxide film formed on the surface of the molten metal is reduced by a reducing action of the magnesium-nitrogen compound to form the surface of the molten metal with pure aluminum, thereby decreasing a surface tension of the molten metal and, accordingly, enhancing a flowing property of the molten metal. As a result, a running property of the molten metal becomes advantageous, whereby a cast product which does not have a cast imperfection but has an excellent appearance without a surface fold or the like can be obtained.
Further, by a subsequent study, according to the reduction casting method, it was found that casting can be performed while holding a temperature of the molding die at low temperature at the time of casting.
Namely, since the flowing property and the running property of the molten metal become extremely advantageous when the reduction casting method is adopted, it is not necessary to hold the temperature of the molding die at high temperature different from other casting methods such as a gravitational casting method (GDC). A reason why the molding die is held warm at the time of casting in the gravitational casting method and the like is to secure the flowing property of the molten metal which fills the cavity by elevating the temperature of the molding die as high as possible. On the other hand, the reduction casting method is excellent in the flowing property and the running property of the molten metal, whereby a filling operation of the molten metal into the cavity is completed in a few seconds. Therefore, in the reduction casting method, it is not necessary to hold the temperature of the molding die at high temperature as is done in a conventional casting method. Rather, it is advantageous from the standpoint of capability of shortening a cycle time of casting that the molten metal poured in the cavity is allowed to be solidified as fast as possible by decreasing the temperature of the molding die as much as possible.
However, problems were generated in that, since the solidification speed of the molten metal became faster, the molten metal was solidified before the molten metal went sufficiently around in the cavity.